Trace gas detection relies on vapors emitted from the explosive or on explosive particles that are deposited on nearby surfaces. Optical detection of explosives is a very challenging task because of complexity of optical spectra of detected materials and their low concentration. Method's sensitivity and selectivity are two main requirements for the detecting technology.
Nguyen et al., in U.S. Pat. No. 6,797,944, disclose a laser desorption and detection of explosives, narcotics and other chemical substances. The Nguyen et al. technique employs a compact scanning apparatus including an optical system to deliver a beam of pulsed infrared laser light that illuminates an interrogation area of the surface. The illumination described by Nguyen et al. is sufficiently intense and of such duration as to cause selective ablation of molecules of contraband substance present on a surface without substantially damaging the surface. A portion or sampling of the ablated molecules is collected and transferred to a separate chemical analysis system where a detector reacts to the sampled portion and activates an audible or visible alarm. A traceable residue of the detected contraband is left on the article for subsequent forensic analysis.
In U.S. Pat. No. 4,591,718 by Amer a sequence of infrared light pulses of progressively changing wavelengths is directed into an interrogated region and additional probe light beam is directed along the sample surface adjacent the region. Infrared wavelengths at which strong absorption occurs in the region are identified by detecting the resulting deflections of the probe beam caused by thermally induced index of refraction changes in the air or other medium adjacent the region.
Remote trace gas detection and analysis are disclosed in U.S. Pat. No. 6,531,701 by Chou. A beam of electromagnetic radiation is used to radiate a cloud and thermalize it by collisional energy transfer between the molecules that absorb the radiation. Emission from the cloud is resolved by a spectrometer.
It is well known that dogs are extremely sensitive to various smells. They have an ability to distinguish a particular explosive from a variety of other confusing smells. This unique selectivity combined with extremely high sensitivity (500 parts per trillion) make them exceptional helpers for security departments.
Most substances targeted for detection by working dogs (e.g., explosives and illicit drugs) are composed of many compounds. Dogs learn to use one constituent compound or combination of constituent compounds to recognize a particular substance. These compounds are called the dog's detection odor signature for that substance.
The present invention discloses an optical method for chemical detection based on trace gas detection similar to sniffing dog's odor signature detection manner.
Artificial olfactory methods were used in U.S. patent application No. 20010041366 by Lewis to detect trace level of analytes in patients' breath for determination of certain medical conditions.
Various optical methods have been developed to detect hidden explosives and other chemicals. The present invention discloses a system and method for remote chemicals detection using coherent detector based on opto-electronic device called 90-degrees optical hybrid. The details on coherent detector design and operating principle are unveiled in co-pending U.S. patent application Ser. No. 10/669,130, filed Sep. 22, 2003, all of which is fully incorporated herein by reference. Similar device is disclosed in U.S. Pat. No. 5,060,312 by Delavaux.
This invention claims benefit of co-pending U.S. patent application No. 20050105099 which discloses remote chemical sensing using photothermal interferometry with coherent detection of an optical signal. Standoff Explosive Detection Using Raman Induced Photothermal Interferometry provides safe identification of explosives at distances from 1 meters to 1000 meters with sensitivity as high as parts per billion. Detection time is less than one second. Low false alarm rates are achieved using multiple laser lines and fast processing algorithms. Principles of photothermal interferometry sensing are disclosed in U.S. Pat. No. 5,365,065 by Power and in Bialkowski S. E. “Photothermal spectroscopy methods for chemical analysis”, John Wiley & sons, 1996, p. 331. None of these references disclose remote sensing, which is crucial for explosives detection. Remote sensing can only be achieved by implementation of modern integrated receivers as disclosed in U.S. patent application Ser. No. 10/669,130.
While the description of the present invention is focused on explosives detection, its use is not limited to this application. A wide variety of commercial applications are available including, but not limited to, environmental toxicology and remediation, biomedicine, such as microorganism classification or detection, material quality control, food and agricultural products monitoring, heavy industrial manufacturing, natural gas leakage detection, ambient air monitoring, worker protection, emissions control, and product quality testing.